Insole. The insole is the foundation of the shoe. In running shoes, it is the part of the shoe upon which the sock liner rests and is commonly referred to as the last. The softer the insole, the less torsional stability the shoe will have. The firmer the insole, the more structure and stability a shoe will have.

Midsole and outsole. The midsole is designed to provide cushioning and shock absorption. The outsole is the part of the shoe that touches the ground and is commonly referred to as the sole.

Running shoes have a thick midsole. In contrast, racing flats, which are designed to be lightweight, have a thin midsole. Midsoles are typically made of ethyl vinyl acetate (EVA), which is the same material often used for orthotic topcovers, only firmer. In general, gray midsole EVA is firm and white EVA is soft.

The firmer the midsole, the stiffer and heavier it will make the shoe. Soft midsoles, on the other hand, improve shock absorption but wear more quickly. Firm midsoles are generally favored for plantar fasciitis, Achilles tendonitis, posterior tibial tendonitis, hallux limitus/rigidus and most other conditions related to excessive pronation. Soft midsoles can be helpful for shin splints due to tibial varum and metatarsalgia.